Non Manifold geometry preventing loop cuts
Downloaded the file, here one solution:



This is only one possible solution but can be applied to both sides of the geometry, you can do other ways, but that depends on what you prefer. That said, it's important to remember that "Face Orientation", picture below lower part, helps to find the problems.



Faces shown as blue represent the outside of the volume of the geometry, red though should only be seen if you literally go between the walls. Can be seen at the end of the gif, showing "one" solution.